Wide-field corrector for a Gregorian telescope

Abstract

A prime-focus corrector for a Gregorian system is proposed that provides a field of view of up to 3° with sub-arsecond image quality at 0.35–0.90 microns. The corrector includes five lenses made of the same glass (preferably fused silica). The distinctive feature of the corrector is the different uses of the central and edge zones of the first lens, which is located at the exit pupil of the two-mirror system. An f/1.9 telescope with a 6.5-m aperture and a total length of 8.8 m is considered as an example. Its primary and secondary mirrors are pure ellipsoids that are close to a paraboloid and sphere, respectively. In the basic configuration, all the surfaces of the corrector are spherical. The diameter of a stellar image D ₈₀ varies from 0″.25 on the optical axis to 0″.50 at the edge of the 2°.3 field. The fraction of vignetted rays grows by 1.7% from the center to the edge of the field of view. Only slightly worse images are given by the spherical corrector for a 2°.4 field of view. Aspherization of some surfaces of the corrected enables sub-arcsecond imaging in a field of view of 3°.0.